1. Field
One embodiment of the invention relates to a perpendicular magnetic recording medium for use in a hard disk drive or the like using the magnetic recording technique and a magnetic recording/reproducing apparatus.
2. Description of the Related Art
A magnetic recording device (HDD) for information recording/reproduction mainly used in computers is used in various fields such as household video decks, audio apparatuses, and car navigation systems because of its large capacity, inexpensiveness, high data access speed, and high data holding reliability. As the range of use of the HDD widens, demands for high storage capacity are increasing, and this has accelerated the competition for the development of a high-density HDD in recent years.
Presently commercially available magnetic recording/reproducing apparatuses use a longitudinal magnetic recording method. In this method, magnetic crystal grains forming a perpendicular magnetic recording layer for recording information have an axis of easy magnetization in a direction longitudinal to a substrate. The axis easy magnetization is an axis in the direction of which magnetization easily points. In a Co-based alloy, the axis of easy magnetization is a direction longitudinal to the normal to the (0001) plane of a hexagonal close-packed structure (hcp) of Co. Decreasing the recording bit area of a longitudinal magnetic recording medium in order to increase the recording density may make the magnetization reversal unit diameter of a recording layer too small. This may worsen the recording/reproduction characteristics by the so-called thermal decay effect which thermally erases information in the magnetic layer. In addition, increasing the density often increases noise produced from the medium by the influence of a demagnetizing field generated in the boundary region between recording bits.
By contrast, in a so-called perpendicular magnetic recording method in which the axis of easy magnetization in a perpendicular magnetic recording layer points in a direction substantially perpendicular to a substrate, the influence of a demagnetizing field between recording bits is small even at high density, and the operation is magnetostatically stable even at a high density. Therefore, the perpendicular magnetic recording method is recently attracting a great deal of attention as a technique which replaces the longitudinal recording method. A perpendicular magnetic recording medium generally comprises a substrate, an alignment control underlayer which aligns magnetic crystal grains in a perpendicular magnetic recording layer in a (0001) plane and reduces the alignment dispersion, the perpendicular magnetic recording layer containing a hard magnetic material, and a protective layer which protects the surface of the perpendicular magnetic recording layer. In addition, a soft magnetic layer which concentrates magnetic flux generated from a magnetic head during recording is formed between the substrate and alignment control underlayer.
To increase the recording density, noise must be reduced with the thermal stability being maintained in this perpendicular magnetic recording medium as well. The noise reduction method is generally a method which decreases the size of the magnetic crystal grains in the recording layer.
An example of the method of reducing this magnetic interaction is a method which adds SiO2 and the like to a perpendicular magnetic recording layer to give it a granular structure formed by magnetic crystal grains and a grain boundary region surrounding the magnetic crystal grains and made of the additives, as described in Jpn. Pat. Appln. KOKAI Publication No. 2002-83411.
Unfortunately, these methods alone cannot sufficiently downsize and magnetically isolate the magnetic crystal grains in the recording layer. In addition to these methods, therefore, there is a disclosed technique which further downsizes the crystal grains in the perpendicular magnetic recording layer by downsizing the crystal grains in the underlayer by adding SiO2 and the like to it, as described in Jpn. Pat. Appln. KOKAI Publication No. 2003-36525.
If a method like this downsizes the crystal grains in the underlayer, however, the crystallinity and alignment of the underlayer crystal grains generally deteriorate. The influence of this deterioration degrades the crystallinity and (0001) alignment of the crystal grains in the perpendicular magnetic recording layer. As a consequence, the signal-to-noise ratio (SNR) decreases.